The present invention relates generally to a label detection and registration system for determining the position or dimension of a label or for counting labels carried on a web material bearing a plurality of such labels, and more particularly, to a label detection and registration or counting system having a capacitance sensor probe assembly.
Label detection and registration systems are used on bottling or packaging lines to insure accurate and repeated registration or placement of a label on a bottle or on other packages. Accuracy of label placement is crucial to maintain a consumer perception of product quality.
Optical techniques using photo-electric sensors have traditionally been used to detect labels carried on a release liner or web backing material. These optical sensors work well for the standard label construction having a web of one color and a label of another color. However, optical sensors have limitations when detecting labels which do not contrast with the backing material (web) and which are constructed of a non-reflective material. These limitations cause errors and sometimes complete failure when detecting a clear or transparent label affixed to a clear or transparent web, or when detecting a label of the same color as the web material. Since there is a significant trend today toward using transparent webs and labels, the attempts to overcome the optical limitation have involved printing registration lines on the clear labels and/or along the entire web. These attempts are costly, inconvenient and aesthetically undesirable.
Other methods of label detection involve mechanical switch methods, but these methods may be unreliable and can damage the label stock.
Capacitive sensors have also been used for label detection by measuring the difference in the dielectric material of the web material alone and the combined web and label material between two parallel electrode plates. The change in capacitance provides an indication of the leading and trailing edges of the label. The capacitive approach is superior to the optical detection techniques because of its substantial independence of the web and label material type, color and opacity. For parallel plate capacitors, the capacitance between the plates is determined using the following equation: ##EQU1## where "C" is the capacitance; ".epsilon..sub.o " is the permittivity of a vacuum; ".epsilon..sub.r " is the permittivity of material between the parallel plates; "S" is the surface area of the plates, and "l" is the distance between the plates.
Capacitance sensors for label registration and detection are known. These known sensors were usually circular, or, if elongated in shape, were too small to be effective and accurate in the label detection environment (e.g., less than a half inch in any dimension). The small size and circular shape of the early sensors resulted in the sensors being insensitive to the edge of the label material and thus caused misalignment of the label. The small surface area "S" also required the air gap distance "l" between the sensor electrode and the opposing metal surface to be very small which often caused the labels to jam under the sensor.
Rectangular sensors have also been used for label detection and registration. More particularly, a single elongated rectangular sensor including a capacitance shield around an active sensing tip is known in the art. However, there is a continuing need for an improved capacitance sensor for use in label detection or in other detection schemes.